Effect of Al doping on the Structural and Magnetic Properties of Iron
Oxide Thin Films
Aseya Akbar1), Sheza Zafar1), Saira Riaz2) and Shahzad Naseem2)
1), 2)
Centre of Excellence in Solid State Physics, University of the Punjab, Pakistan
*)
[email protected]
ABSTRACT
During the past few decades iron oxide nanostructures are gaining worldwide
attraction due to its applications in various industrial and medical areas. This is due to
the unique structural, magnetic and electrical properties of iron oxide that it has found
its usage in water purification, biosensors, magnetic resonance imaging, drug delivery
etc. In addition iron ion in iron oxide can be replaced by other metals to enhance its
certain properties for different practical applications. Al doped iron oxide thin films are
prepared by thermal evaporation method. The concentration of aluminum was varied
from x=0 to x=1 in Fe2-xAlxO4. XRD results show the formation of highly crystalline iron
oxide undoped and thin films. The ionic radii of iron Fe +3 is 0.069nm and Al+3 is
0.0675nm thus, incorporation of aluminum in iron oxide lattice caused slight shrinkage
of unit cell thus leading to very little decrease in lattice parameter. No peaks
corresponding to Al2O3 was observed in XRD indicating that aluminum is successfully
incorporated within the host lattice in the concentration range studied with decrease in
peak intensities with increasing Al+3 content. The band gap of the films was studied
using Variable Angle Spectroscopic Ellipsometer (VASE) and is found in the range of
2.14 to 2.55eV. Moreover, aluminum doping also strongly affect the magnetic and
dielectric properties of iron oxide thin films.
1. INTRODUCTION
During the past few years, the controlled growth of spinel nanostructures with
desired morphology is of technological interest due to its exciting properties like Giant
MagnetoResistive (GMR) tunneling and electric field effects (Yanagihara 2013, Wu
2012). These properties have found wide applications in spintronic devices (e.g.
Magnetic Tunnel Junctions (MTJ’s)), humidity sensors, in Magnetic Resonance Imaging,
targeted drug delivery, magnetic recording etc. Spinel oxides appear flexible due to
their complex structure and the resulting high degrees of freedom (Ikeda 2007). The
crystal structure of spinels XFe2O4 (X= Co, Mg, Mn, Ni etc) is cubic. There are 8 corner
1)
2)
Graduate Student
Professor
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atoms in cubic unit cell. Each corner is occupied by ferrite molecules and resulting in
eight ferrite unit cells. Therefore, there are 8 ferrous (Fe+2) ions, 16 ferric (Fe+3) ions
and 32 oxygen (O-2) ions in a ferrous ferrite unit cell. If oxygen atoms are considered
alone it constitutes a FCC structure. There are sixteen octahedral sites and 8
tetrahedral sites in a spinel ferrite unit cell (Liu 2013, Teresa 2000).
To date various spinel structures of the form XFe2O4 has been reported including
CoFe2O4 (Cui 2010), MgFe2O4 (Chandradass 2013), MnFe2O4 (Vamvakidis 2013),
NiFe2O4. Soderlind (2009) found the presence of impurity garnet Gd3Fe5O12 phase in
perovskite GdFeO3 films. The magnetization of GdFeO3 was attributed to two factors
one arising from iron sublttice and other from gadolinium sublattice. Gupta (2011)
prepared MgFe2O4 using Pulsed Laser deposition and obtained ferromagnetic behavior
of the films at room temperature.
Among the inverse spinel structures of the form XFe2O4 (where X= Co, Mg, Mn, Ni
etc.) very little attention has been given to AlFe 2O4. AlFe2O4 is a normal spinel oxide in
which Fe+2 cations occupy 1/8th of the tetrahedral sites and Al+3 occupy ½ of the
octahedral sites. The distribution of charges on octahedral and tetrahedral changes
depending on the synthesis conditions as octahedral sites can also be occupied by
Fe+2 cations.
We here prepared the much neglected spinel ferrite AlFe 2O4 using thermal
evaporation of iron and aluminum followed by oxidation under at 10sccm at 150˚C450C.
2. EXPERIMENTAL DETAILS
Preparation of AlFe2O4 was done using thermal evaporation method in Edward 306
coating unit. Before deposition of thin films the corning glass substrates were placed in
acetone in ultrasonic bath for ten minutes followed by ultrasonication in Isopropyl
Alcohol (IPA) for 15mins. Iron and aluminum were evaporated by sequential elemental
layer technique at a base pressure of 10-6 torr and working pressure of 1.5×10-5 torr
and then oxidized in the presence of oxygen flow of 10sccm at 150˚C, 250˚C and
350˚C for 60min. The flow rate of oxygen is chosen on the basis of work done earlier by
Riaz (2013). Structural characterization and optimization is done by XRD (Rigaku DMAX II A Diffractometer) and magnetic properties are studies by VSM (Lake Shore
7407 Magnetometer).
2. RESULT AND DISCUSSIONS
Fig. 1 show XRD patterns for AlFe2O4 thin films deposited on glass substrate and
annealed at 150˚C, 250˚C and 350˚C. The films show amorphous behavior under as
deposited conditions where as annealing does not affect the crystalline structure of the
films. However the presence of very small diffraction peaks indicates the formation of
the required AlFe2O4 phase at oxygen flow rate of 10sccm. The spinel ferrites of the
form XFe2O4 (where X=Co, Mg, Mn, Ni etc.) is of technological importance owing to its
spintronic applications. However the spinel ferrite AlFe2O4 is long neglected and to the
best of our knowledge there are no reports so far on AlFe2O4 spinel ferrite.
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Fig. 1 XRD pattern of AlFe2Ox thin films oxidized at oxygen flow rate of 10sccm and
annealed at 150˚C, 250˚C and 350˚C.
Fig. 2 show XRD room temperature in-plane and out-plane magnetization curve of
AlFeOx film annealed at 350˚C. The film show ferromagnetic behavior with high
coercivity. The saturation magnetization of the film for in-plane and out-plane applied
field is 0.002 and 0.0015 respectively. The coercivity of the films for in-plane and outplane field is 1980Gauss and 1995Gauss respectively.
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Fig. 2 Room temperature M-H curve for AlFeOx thin film annealed at 350˚C.
3. CONCLUSIONS
Spinel oxide AlFeOx has been investigated for its structural and magnetic properties.
The films are deposited using thermal evaporation of iron and aluminum followed by
annealing at 150˚C, 250˚C and 350˚C in an oxygen flow of 10sccm. XRD results show
that films exhibit amorphous behavior with very small diffraction peaks. The film show
ferromagnetic properties of film annealed at 350˚C.
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